System and method for odorizing natural gas

ABSTRACT

The present disclosure provides a system and method for odorizing natural gas flowing through a distribution pipeline. The system includes a bypass line adjacent to a distribution pipeline, wherein bypass gas flows through the bypass line and an odorant tank connected to the bypass line, and into the distribution pipeline; a high-flow control valve and a low-flow control valve in the bypass line, wherein bypass gas flows through the odorant tank into the distribution pipeline when the high-flow control valve or the low-flow control valve is open; and a programmable logic controller connected to the high-flow and low flow control valve; wherein the programmable logic controller opens the high-flow or low-flow control valve for a predetermined dwell time proportional to an amount of bypass gas needed to odorize gas in the distribution pipeline each time that a preselected quantity of gas flows through the distribution pipeline.

BACKGROUND

The present invention relates generally to odorization of natural gas,and more specifically to an improved system and method for addingodorant to natural gas flowing in a pipeline.

Natural gas is odorless. Because of its potentially dangerous nature,for many years federal regulations have required the addition of anodorant to natural gas so that it can be detected by smell. Odorantssuch as tertiary butyl mercaptan (TBM) and various blends of commonlyaccepted chemicals are used in the industry.

The odorants added to natural gas, which are provided in liquid form,are often added to the gas at the location where distribution gas istaken from a main gas pipeline and provided to a distribution pipeline.At this point, the gas pressure is stepped down through a regulator,typically from a pressure of approximately 600 psi or more to a lowerpressure of approximately 100 psi or less. The odorants can also beadded to gas in the main transmission pipeline.

Odorants used with natural gas are extremely concentrated, so that onlya small amount of liquid is needed to odorize a relatively large volumeof natural gas. For example, with odorants such as TBM and other blends,it is common to use approximately 0.75 lbs. of liquid odorant to odorize1,000,000 standard cubic feet (SCF) of natural gas.

Odorants such as TBM and other blends are mildly corrosive, and verynoxious. It is important that a correctly measured amount of odorant beadded to natural gas; otherwise, various problems will result. Forexample, over-odorization results in excess odors within the valves,pipes, and other equipment used in natural gas distribution. Inaddition, too much odorant causes the distinctive odorant smell to benoticeable even after the natural gas is burned. This leads to consumercalls complaining of natural gas leaks, each of which must be respondedto by the natural gas distribution company. The expense of such calls,when there is no leak involved, is quite high.

It is also important that the odorant levels not be too low. Safetyconsiderations mandate that a natural gas leak be easily detectable bymost people. The proper concentration of odorant within natural gasprovides this safety measure, but under-odorization is dangerous becauseactual leaks may not be detected in time.

One technique in current use to provide odorization to natural gas in amain distribution pipeline involves bypassing a small amount of naturalgas, at a slightly higher pressure than the low pressure distributionpipeline, through a tank containing liquid odorant. This bypass gasabsorbs relatively high concentrations of the odorant while in the tank.When this heavily odorized bypass gas is placed back into the mainpipeline, the odorant, now in gas form, diffuses throughout thepipeline.

Because the bypass gas picks up such large amounts of odorant from theliquid in the tank, becoming completely saturated with odorant gas, itis necessary that carefully monitored small amounts of bypass gas beused. The present approach is to divert the bypass gas through onesolenoid valve controlled by a programmable logic controller (PLC). ThePLC causes the solenoid valve to open and close for periods of timeranging from fractions of a second to several minutes, depending on thepressure of the bypass gas coming in and the desired pressure comingout. Ideally, the solenoid will close and open fairly often so that theflow of gas is relatively stable. For example, the solenoid valve may beopen for 50% of every 10 second period. However, the single-solenoidvalve system can be problematic when consumer demand for natural gaschanges, such as during seasonal changes. During winter, demand fornatural gas is much higher, whereas during summer, the demand isminimal. A single solenoid may have to remain open for very longstretches of time in one season (e.g., ≥90% during winter) and for veryshort periods of time in another season (e.g., ≤10% during summer),which results in an unstable flow of gas.

It would be desirable to provide an odorization technique, and equipmentto be used therewith, which provides both a more reliable and flexibletechnique for odorizing natural gas. It would be desirable that theequipment be simple and reliable in operation, and that it be flexibleenough to easily accommodate significant changes in the operatingcondition of the gas pipeline and changes in consumer demand for naturalgas.

SUMMARY OF THE INVENTION

The present disclosure provides a system for odorizing natural gasflowing through a distribution pipeline. The system includes:

a bypass line adjacent to a distribution pipeline;

an odorant tank connected to the bypass line, wherein bypass gas flowsthrough the bypass line, through the odorant tank, and into thedistribution pipeline;

a high-flow control valve and a low-flow control valve in the bypassline, wherein bypass gas flows through the odorant tank and into thedistribution pipeline only when (i) the high-flow control valve is open,(ii) the low-flow control valve is open, or (iii) both the high-flowcontrol valve and the low-flow control valve are open;

a flow meter on the distribution pipeline that monitors gas flow throughthe distribution pipeline; and

a programmable logic controller connected to the flow meter, thehigh-flow control valve and the low-flow control valve;

wherein the programmable logic controller opens (a) the high-flowcontrol valve, (b) the low-flow control valve, or (c) both the high-flowcontrol valve and the low-flow control valve, for a predetermined dwelltime that is proportional to an amount of bypass gas needed to odorizegas in the distribution pipeline each time that a preselected quantityof gas flows through the distribution pipeline.

The present disclosure also provides a controller for a natural gasodorant system. The controller includes:

an input adapted to accept an input signal from a gas flow meter;

an output adapted to control a high-flow control valve and a low-flowcontrol valve;

a memory storing a dwell time value, a target flow count valueindicative of how often the high-flow control valve or the low-flowcontrol valve should be opened with relation to gas flow, and a targetopening duration value indicative of whether to open (a) the high-flowcontrol valve, (b) the low-flow control valve, or (c) both the high-flowcontrol valve and the low-flow control valve; and

an output driver for sending a signal from the output to (a) thehigh-flow control valve, (b) the low-flow control valve, or (c) both thehigh-flow control valve and the low-flow control valve, for a timeperiod equal to the stored dwell time, wherein the signal is sent fromthe output at times determined by the target flow count value and datareceived through the input, and the signal is sent to either thehigh-flow control valve or the low-flow control valve at timesdetermined by the target opening duration value.

The present disclosure also provides a method for odorizing natural gasin a pipeline. The method includes the steps of:

providing liquid odorant in a tank;

measuring gas flow through the pipeline;

measuring a valve opening duration of a high-flow control valve in abypass line and a low-flow control valve in the bypass line;

comparing the gas flow with a target flow count;

comparing the valve opening duration with a target valve openingduration;

opening the high-flow control valve when (i) the gas flow is equal to orgreater than the target flow count and (ii) the valve opening durationis equal to or greater than the target valve opening duration, to allowa first known volume of gas, saturated with odorant, to flow from theodorant tank into the pipeline; wherein the high-flow control valve isopened for a dwell time proportional to the first known volume ofsaturated gas; and

opening the low-flow control valve when (i) the gas flow is equal to orgreater than the target flow count and (ii) the valve opening durationless than the target valve opening duration, to allow a second knownvolume of gas, saturated with odorant, to flow from the odorant tankinto the pipeline; wherein the low-flow control valve is opened for adwell time proportional to the second known volume of saturated gas.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a natural gas odorization systemconstructed in accordance with an embodiment of the present disclosure;

FIG. 2 is a flowchart illustrating normal operation of a natural gasodorization system in accordance with an embodiment of the presentdisclosure;

FIG. 3 is a diagram of the odorant tank of the natural gas odorizationsystem of FIG. 1;

FIG. 4 is a flowchart illustrating operation of the programmable logiccontroller in performing temperature compensation in accordance with anembodiment of the present disclosure; and

FIG. 5 is a graph illustrating odorant changes in volume as a functionof temperature changes.

DETAILED DESCRIPTION

As described below, the present disclosure provides a bypass odorizationsystem in which small amounts of odorant saturated gas are added to anatural gas distribution or transmission pipeline. A known,pre-calculated volume of saturated gas is added for a known volume ofgas flowing through the main pipeline. The known volume of gas isobtained by opening one of at least two control valves in the bypassline for a pre-determined time period, referred to herein as the “dwelltime” of a control valve. Because the valve characteristics and pressuredrop across the valves are known, the volume of gas that flows throughthe valves during the dwell time can be accurately pre-calculated. Eachcycle of opening and closing one control valve is referred to as a“pulse.”

Referring to FIG. 1, a gas odorization system 10 is shown schematically.A pressure regulator 12 is used to connect a main pipeline (alsoreferred to as a “high pressure pipeline” or a “main gas line”) 14 witha distribution pipeline 16 (or a transmission pipeline). Gas pressure inthe main pipeline 14 can be, for example, 600-800 psi, while gaspressure in the distribution pipeline 16 is lower. A typical pressure indistribution pipeline 16 is 100 psi.

A tap 18 off the high pressure pipeline 14 is connected to a bypassregulator 20, which in turn allows bypass gas to flow to a low-flowcontrol valve 22 and a high-flow control valve 23. In an embodiment, thebypass regulator 20 is a differential regulator that always provides gasat a pressure differential of 12-40 lbs. above that in the distributionpipeline 16, regardless of any variations that may occur in the gaspressure in the main pipeline 14. A pressure differential of 12-40 psimay be chosen, but it is desirable that the gas supplied to the low-flowcontrol valve 22 and the high-flow control valve 23 be fixed at a knownvalue above the gas pressure in distribution pipeline 16. A differentialregulator may be used to maintain a constant pressure above that in thedistribution pipeline 16.

Other techniques can be used to provide the pressure differential forthe bypass gas line. For example, a compressor can be used to raise thepressure in the bypass line above that in the distribution line 16. Itis still desirable to provide a differential regulator so that thebypass line pressure remains a constant value above that in thedistribution line 16. This maintains the accuracy of gas flow throughthe low-flow control valve 22 and the high-flow control valve 23, as isexplained in more detail below. Any technique that maintains pressureinto the bypass line at a constant, known differential above thepressure in distribution line 16 can be used in this system and iscontemplated by the present disclosure.

The “low-flow control valve” and the “high-flow control valve” areindividual control valves with different gas flow rates. The low-flowcontrol valve 22 has a lower flow rate relative to the flow rate of thehigh-flow control valve. Thus, the high-flow control valve 23 allows arelatively large amount of gas to pass through the bypass line 38 tosatisfy increased consumer demand for gas, such as during winter, whilethe low-flow control valve 22 allows a relatively small amount of gas topass through the bypass line 38 to satisfy decreased consumer demand forgas, such as during summer. In each case, the percentage of time thatthe low-flow control valve 22 or the high-flow control valve 23 is openduring operation of the gas odorization system 10 may remain the same,or substantially the same, which advantageously allows for a flow of gasthat is stable, or substantially stable. In an embodiment, the low-flowcontrol valve 22 and the high-flow control valve 23 each is anelectrically actuated solenoid valve.

In an embodiment, the low-flow control valve 22 has a flow rate from 100cubic inches per second (in³/sec), or 200 in³/sec, or 300 in³/sec to 400in³/sec, or 500 in³/sec. In an embodiment, the high-flow control valve23 has a flow rate from greater than 500 in³/sec, or 550 in³/sec, or 600in³/sec, or 700 in³/sec to 800 in³/sec, or 900 in³/sec, or 1000 in³/sec,or 1500 in³/sec. Because the bypass regulator 20 regulates gas pressurein the bypass line 38 to be a known differential higher than thepressure in the distribution line 16, preferably 12-40 psi higher, theflow rate of gas through the low-flow control valve 22 and the high-flowcontrol valve 23 can be easily calculated from the properties of eachvalve. Valve manufacturers typically provide tables, equations, or otherdata from which the flow rate of gas through the low-flow control valve22 and the high-flow control valve 23 can be determined.

In an embodiment, the bypass line 38 is split (for example, with a Yconnector) downstream of the bypass regulator 20 such that gas may flowto the low-flow control valve 22 or the high-flow control valve 23, andthe split bypass line 38 is combined downstream of the low-flow controlvalve 22 and the high-flow control valve 23 (for example, with a Yconnector).

While the present disclosure describes a gas odorization system 10 withtwo control valves (the low-flow control valve 22 and the high-flowcontrol valve 23), it is understood that the gas odorization system mayinclude more than two control valves, or from 2, or 3 to 4, or 5 controlvalves, wherein each control valve has a different flow rate. The gasodorization system includes at least two control valves. In anembodiment, the gas odorization system includes more than two controlvalves, wherein each control valve has a different flow rate. In anotherembodiment, the gas odorization system includes from 2 to 3, or 4, or 5,or 6, or 7, or 8, or 9, or 10 or more control valves, wherein eachcontrol valve has a different flow rate.

Odorant is stored in a tank 24 in liquid form. Any odorant used forodorizing natural gas, such as tertiary butyl mercaptan (TBM), may beused. An outlet line 26 passes from the tank 24 through a check valve 28and back into distribution pipeline 16.

When the odorant tank 24 is filled with liquid odorant, space is leftfor gas above the surface of the odorant. In operation, this space isfilled with natural gas that has entered the tank 24 through thelow-flow control valve 22 or the high-flow control valve 23. Because ofthe high volatility of the liquid odorant, gas contained in the tank 24above the surface of the liquid odorant is saturated with odorant ingaseous form.

A programmable logic controller (PLC) 30 is used to control operation ofthe gas odorization system 10. The PLC 30 has inputs connected to a flowmeter 32 in the distribution pipeline 16, a temperature sensor 34 withinthe odorant tank 24, and an optional level sensor 36 within the odoranttank 24. As described below, the temperature sensor 34 compensates fortemperature fluctuations within the odorant liquid. The flow meter 32generates a signal proportional to the flow of gas within thedistribution pipeline 16. Preferably, the flow meter 32 provides adigital pulse, or an analog signal, each time a known quantity of gasflows through the distribution pipeline 16. By counting the pulsesprovided by the flow meter 32, or by calculating flow from the analogsignal provided by an analog meter, the PLC 30 can determine when aknown, pre-selected volume of gas has flowed through the distributionpipeline 16.

The temperature sensor 34 measures the temperature of liquid odorantwithin the tank 24. In a typical installation, the tank 24 is locatedabove ground, and temperature variations in accordance with weather andtime of day are common. Liquid odorant contained in the tank 24 expandsas its temperature increases, and contracts as its temperaturedecreases. Compensation for this volumetric change provides properodorization of the gas in the distribution pipeline 16, as describedbelow in more detail.

The level sensor 36 detects the level of liquid odorant remaining in thetank 24. This data is useful to an operator of the odorizer,particularly when the odorizer is monitored remotely, as describedbelow, in order for liquid odorant to be replenished when its levelbecomes low, and so that proper odorant use may be verified.

The PLC 30 also provides an output signal that controls the low-flowcontrol valve 22 and the high-flow control valve 23. In an embodiment,the low-flow control valve 22 and the high-flow control valve 23 each isan electrically actuated solenoid valve, but can alternatively be anyvalve that may be controlled by a suitable signal from the PLC 30.

Once the flow rate for the low-flow control valve 22 and the high-flowcontrol valve 23 is determined (as described above), in advance, thelow-flow control valve 22 and the high-flow control valve 23 need merelybe held open for a calculated interval, referred to herein as the “dwelltime,” in order to allow a known volume of gas to pass through thebypass line 38 and into the odorant tank 24. The gas flowing into thetank 24 will cause an equal volume of saturated gas to flow through theoutlet line 26, and the check valve 28, into the distribution line 16.By simply adjusting the dwell time for the low-flow control valve 22 andthe high-flow control valve 23, the volume of bypass gas injected intothe main pipeline 16 can be easily controlled.

The PLC 30 itself may be any suitable programmable logic controlleravailable in the industry, and may be, for example, a DL250CPUcontroller available from PLC Direct.

As described in connection with FIG. 1, the PLC 30 has inputs connectedto the flow meter 32, the temperature sensor 34, and the optional levelsensor 36. The PLC 30 provides a control signal output to the low-flowcontrol valve 22 and the high-flow control valve 23, each of which ispreferably an electrically actuated solenoid valve. In addition to theseprimary connections, other inputs and outputs are provided for the PLC30.

A power supply provides power for the PLC 30. The power supply may beany suitable supply as known in the art. A backup battery is preferablyprovided to keep the PLC 30 powered in case of a failure in the powersupply. In addition, a solar supply may be used to provide charge to thebackup battery. In suitable conditions, the power supply can bedispensed with and all power for the PLC 30 provided from the solarsupply through the battery, which is especially advantageous forinstallation in remote locations where normal power may not beavailable.

For safety considerations, a master shutdown valve 46 is optionallyprovided in the bypass line 38 along with the low-flow control valve 22and the high-flow control valve 23. The shutdown valve 46 operates inconjunction with the flow sensor 48, and is used only in the case of afailure elsewhere. The shutdown valve 46 is maintained in a normallyopened position during odorizer operation. When either the low-flowcontrol valve 22 or the high-flow control valve 23 is opened (oroptionally, both the low-flow control valve 22 and the high-flow controlvalve 23 are open), the flow sensor 48 should sense a flow of gasthrough the bypass line 38. It is not necessary to measure the volume ofgas flowing through the bypass line 38, but simply to sense itspresence. If the PLC 30 determines that no gas is sensed by the flowsensor 48 while either the low-flow control valve 22 or the high-flowcontrol valve 23 is opened, the master shutdown valve 46 is closed tostop all bypass gas flow. Such a situation could occur with a failure ofthe low-flow control valve 22 and/or the high-flow control valve 23, andclosing the shutdown valve 46 prevents the escape of natural gas out ofthe system. If either the low-flow control valve 22 or the high-flowcontrol valve 23 fails in an open position, the flow sensor 48 will becontinuously activated. Because this condition would cause extremeover-odorization of the gas in the distribution pipeline 16, theshutdown valve 46 can be used to interrupt gas flow through the bypassline 38. In this case, if desirable, the PLC 30 can be programmed totoggle the shutdown valve 46 in response to gas flowing through thebypass line 38—in effect using shutdown valve 46 as a backup controlvalve. In an embodiment, the shutdown valve 46 is an electricallyactuated solenoid valve, but can alternatively be any valve that may becontrolled by a suitable signal from the PLC 30.

In an embodiment, the PLC 30 also provides one or more alarm outputs.These alarm outputs can be used to activate different types of alarms asappropriate and as known in the art. For example, a visible and/oraudible alarm can be provided at the odorizer location. In addition, asignal may be sent to a remote site, via direct connection, radio ortelephone line, to indicate to an operator that a failure has occurredand that immediate action is required. Different types of alarms, bothinformative and emergency, may be provided as desired by the PLC 30. Theprovision of alarms of different types is well known in the art, andwill not be discussed in detail herein.

In an embodiment, the PLC 30 monitors the signal from the flow meter 32to ensure that a signal is being received. If the PLC 30 loses thesignal from the flow meter 32, whether such signal is digital pulses oran analog signal, the PLC 30 assumes that the flow meter 32 is notoperating. At this point, any of three options can be performed, withthe desired option being programmed into the PLC 30 ahead of time by anoperator. The first option is to shut down the odorizer by closing eachof the low-flow control valve 22, the high-flow control valve 23 and theshutoff valve 46, and generate the appropriate alarm. The second optionis to enter a backup mode in which either the low-flow control valve 22or the high-flow control valve 23 is operated at a constant rate that isprogrammed in advance, while activating the appropriate alarms. Thethird option is similar to the second, except that the average gas flowrate over a previous interval of time, for example one or two hours, isassumed to remain constant. The low-flow control valve 22 or thehigh-flow control valve 23 is then operated at a constant rateappropriate for that flow. As before, an appropriate alarm is generated.

The PLC 30 is also provided with at least one, preferably two,input/output data connections. A keypad, and an integral associateddisplay, is physically located within the gas odorization system 10 forlocal access to the system by an operator. The PLC 30 can bereprogrammed through the keypad, and its status reviewed on theassociated display.

In an embodiment, a computer is connected to the PLC 30 through a secondinput/output connection. This connection can be a direct serial link asknown in the art, or may be made over a telecommunications line throughmodems (associated with the PLC 30 and/or associated with a remotecomputer). Through the use of modems connected over a phone line, thecomputer can be located anywhere. Use of a computer provides for greaterease in programming the PLC 30 and in collecting data generated by andstored within the PLC 30. Preferred methods for utilizing a remotecomputer in conjunction the PLC 30 will be described in more detailbelow. When a direct serial link is available, the computer can be, forexample, a laptop computer carried to the odorization site.

Most of the time, the gas odorization system 10 will be functioning inits normal operating mode. FIG. 2 is a flowchart describing the normaloperating mode for PLC 30.

As gas flows through the distribution pipeline 16, a signal proportionalto gas flow is generated by the flow meter 32. This signal is preferablydigital pulses, or an analog signal that is converted to pulses for easycounting by the PLC 30. These pulses are detected 60 within the PLC, andfor each pulse detected, a flow counter is incremented 62. The valuestored within the counter is compared with a target flow count 64.

The target flow count is a value selected in advance as a function ofboth the properties of the flow meter 32, and the desired volume of gasflow through the distribution pipeline 16 that is desired to trigger acycle of the gas odorization system 10. For example, if the flow meter32 generates a pulse for every 10 standard cubic feet (SCF) of gas flow,and a cycle of odorizer is desired for every 200 SCF, the target flowcount can be set to 20. If the flow counter value is less than thetarget flow count, the PLC returns to step 60 to await another flowpulse.

Once the counter reaches the target count in step 64, a valve openingduration, as described below, is compared with a target valve openingduration 65.

The target valve opening duration is a value selected in advance as afunction of the desired continuity of the gas flow through thedistribution pipeline 16. Ideally, either the low-flow control valve 22or the high-flow control valve 23 will close and open fairly often sothat the flow of gas is relatively stable (i.e., relatively continuous).In an embodiment, the target valve opening duration is 50%—indicatingthat either the low-flow control valve 22 or the high-flow control valve23 is in an open position for 5 seconds of every 10 second period.

If the valve opening duration is equal to or greater than (≥) the targetvalve opening duration, the high-flow control valve 23 is opened for thepre-calculated dwell time (step 66 a), the valve opening duration isincremented 67, and the flow pulse counter is reset 68. In anembodiment, when the valve opening duration is greater than (>) thetarget valve opening duration, the high-flow control valve 23 and thelow-flow control valve are both opened for the pre-calculated dwelltime.

If the valve opening duration is less than (<) the target valve openingduration, the low-flow control valve 22 is opened for the pre-calculateddwell time (step 66 b), the valve opening duration is incremented 67,and the flow pulse counter is reset 68. In an embodiment, if the valveopening duration is less than (<) the target valve opening duration, thelow-flow control valve 22 is opened, and the high-flow control valve 23is not opened.

The valve opening duration is incremented 67 such that the combinedduration of time that the low-flow control valve 22 and the high-flowcontrol valve 23 are open is provided as a percentage of time. In anembodiment, the control valve opening duration is incremented 67 in 10second periods such that the control valve opening duration incremented67 is the percentage of time within the 10 second period immediatelyprior to the moment valve opening duration is incremented that thelow-flow control valve 22 and the high-flow control valve 23 are open.

In an embodiment, the odorizer 10 maintains information indicating itsoperational history. This information is advantageous for variousreasons, including for oversight of the operation of the equipment, andfor historical purposes in case of an accident. Audit information thatis saved typically includes data showing the volume of odorant placedinto the distribution pipeline 16, and similar information such as gasflow through the distribution pipeline 16 and the level of odorantremaining in the tank 24. This information is typically updated in step70, and stored within the memory of the PLC 30. After the audit reportinformation is updated 70, control returns back to step 60 to await thenext pulse from the flow meter 32.

In an embodiment, the present bypass technique provides compensation forchanges in temperature of the liquid odorant. FIG. 3 illustrates thebenefits of such temperature compensation.

As shown in FIG. 3, bypass gas flows into the odorant tank 24 throughthe bypass line 38, and flows out of the odorant tank 24 to the gasdistribution pipeline 16 through the outlet line 26. The tank 24contains a quantity of liquid odorant 72, leaving a space 74 above anupper surface level 76 of the odorant 72 (when the temperature of theodorant 72 is constant, the upper surface level 76 does not change).Thus, when a given volume of gas flows into the tank 24 through thebypass line 38, an equal volume flows out of the tank 24 through theoutlet line 26 (less the amount of odorant displaced).

The liquids used as odorants for natural gas are relatively sensitive totemperature changes. When the temperature of the liquid increases, sodoes its volume. When the temperature decreases, its volume decreaseslikewise. This change in volume with temperature is advantageouslycompensated by the PLC 30.

Referring to FIG. 3, assume that upper surface level 76 represents theupper surface level 76 of liquid odorant 72 at a given temperature. Ifthe temperature of the liquid odorant 72 increases, such as can happenwhen direct sunlight shines on the odorant tank 24, its volume expands.After the expansion, the upper surface level of the odorant is at levellocation 78. Because the odorant 72 has expanded to fill the volumebetween surface levels 76 and 78, gas equivalent to this volume isforced out of the tank 24. Because of the setup of the gas odorizationsystem 10, as illustrated in FIG. 1, this excess gas, which is saturatedwith odorant in the gas phase, will flow through the outlet line 26 andinto the distribution pipeline 16.

Thus, as a result of liquid odorant 72 expansion, additional odorizedbypass gas has been placed into the distribution pipeline 16. Unlesscompensated for, this will cause over-odorization of the gas in thedistribution pipeline 16. In an embodiment, the odorizer 10, and furtherthe PLC 30, compensates for such a temperature increase by skipping oneor more odorizing pulse cycles normally caused by gas flow through thedistribution line 16.

In a similar manner, if the initial level of the odorant is as indicatedby upper surface level 78, and the liquid odorant 72 cools, its volumewill diminish. Cooling can be caused by, for example, night or cloudyweather causing an ambient temperature change. As the upper surface ofthe odorant 72 falls from level 78 to level 76, the volume change mustbe made up by additional gas in the space 74.

Because of the check valve 28 on the outlet line 26, gas cannot flowback into the tank 24 through the outlet line 26. In addition, becauseof the valves on the bypass line 38, additional gas cannot flow into thetank 24 through the bypass line 38. Thus, the pressure in the tank 24will decrease slightly when the volume of the liquid odorant 72decreases. This slight decrease in pressure will eventually be made upby gas flowing into the tank 24 through the bypass line 38 when eitherthe low-flow control valve 22 or the high-flow control valve 23 isopened, but may take one more pulses of either the low-flow controlvalve 22 or the high-flow control valve 23 to raise the gas pressurewithin the tank 24 and again enable gas to flow out through the outletline 26.

In an embodiment, this loss of volume due to a lowering of the uppersurface level 76 of liquid odorant 72 is compensated for by executingone or more additional pulses of the low-flow control valve 22 or thehigh-flow control valve 23. This will supply extra gas into the space 74to make up for the loss in volume caused by the cooling of the odorant72. Consequently, the PLC 30 compensates for changes in odoranttemperature by modifying the dwell time of the high-flow control valve23 and the low-flow control valve 22.

Regardless of the temperature of the liquid odorant 72, so long as thetemperature remains constant, the volume will remain constant, exceptfor the very small decreases caused when new gas enters the tank 24 andabsorbs odorant. Thus, compensation by way of extra pulses, oreliminating pulses, need only be performed when the temperature changes.

FIG. 4 is a flowchart illustrating operation of the PLC 30 to compensatefor temperature changes in the liquid odorant 72. As shown in step 80,the liquid odorant 72 temperature is constantly monitored. A check ismade to see if the temperature of the liquid odorant 72 has increased bya sufficient temperature 82. If not, a check is made to see if theliquid odorant 72 temperature has decreased by a sufficient change intemperature 84. If not, the PLC returns to its monitor cycle 80.

If the temperature of the liquid odorant 72 has gone up by a sufficientamount (step 82), additional odorant has been injected into thedistribution pipeline 16 by the volume increase of the liquid odorant72. Therefore, the PLC 30 skips the next odorant pulse 86 by resettingthe flow pulse counter when the gas odorization system 10 reaches thetarget flow count without opening the low-flow control valve 22 or thehigh-flow control valve 23 for the pre-calculated dwell time.

If the result of step 84 indicates that the liquid odorant 72 has cooledby a sufficient amount, additional gas needs to be supplied to theinterior of odorant tank 24. The PLC 30 therefore performs an extrapulse immediately 88 by opening either the low-flow control valve 22 orthe high-flow control valve 23 (depending on the valve opening duration)for the calculated dwell time without resetting the flow pulse counter.This will cause an extra volume of gas to be injected into the odoranttank 24 to make up for the change in volume of the cooling liquidodorant 72.

As an example of the temperature compensation, assume that a pulse is tobe generated for every 100 standard cubic feet of gas that flows throughthe line. Further assume that 100 cubic inches of saturated gas providesthe required level of odorant for 100 standard cubic feet of gas. Whenthe temperature rises enough that the volume of the liquid odorantincreases by 100 cubic inches, one pulse will be skipped. When thetemperature falls by the same amount, an extra pulse will be added.

As the temperature of the liquid odorant 72 rises and falls, pulses willbe occasionally skipped and added respectively. The actual volumetricchange that occurs as a result of temperature variations is a functionof both the change in temperature and amount of odorant in the tank 24.Changes in volume are a percentage of total liquid volume, so differentodorant levels will see different absolute volume changes for a giventemperature change.

Any suitable technique can be used to compensate for temperaturechanges. A simplifying assumption can be made regarding volume, so thatpulses are generated or skipped regardless of the actual volume ofliquid odorant currently in the tank. If desired, the output from thelevel sensor 36 can be incorporated into the calculation, so that actualvolumetric changes are used to determine when to add or subtract pulses.It is also possible to vary the dwell time of the low-flow control valve22 and/or the high-flow control valve 23, or adjust the timing betweenpulses, to compensate for temperature changes. Whether changing thenumber of pulses, the dwell time of the low-flow control valve 22 or thehigh-flow control valve 23, or the timing between pulses, compensatingfor temperature changes in the liquid odorant 72 is desired to provideproper odorization levels.

FIG. 5 is a graph illustrating how the liquid odorant volume varies withtemperature. This particular graph is for odorant BP CAPTAN (whichcontains TBM as the principal odorant) between the temperatures of −30°F. and 120° F. It indicates a correction factor to be applied to obtainthe real volume of the liquid odorant in compensation for temperaturevariations. It is normalized for a temperature of 60° F. This data iswidely available, and differs for different materials. The graph shownis for illustrative purposes to show how temperature compensation isperformed in an embodiment of the present disclosure.

Because BP CAPTAN liquid odorant expands as its temperature rises, themeasured volume of liquid is multiplied by a smaller number to give truevolume. For example, at 120° F., the measured volume must be multipliedby 0.95 to arrive at the actual volume that would be measured at 60° F.

As shown by the straight line on the graph of FIG. 5, volumetric changesare essentially linear. Over a temperature range of 150° F., the volumeof a given amount of liquid odorant will change by more than 10%. Usingthe values from this graph, it will be appreciated that the liquidodorant will change in volume by approximately 0.00083% for each degreechange in temperature. For a liquid volume of 20 cubic feet, thisamounts to approximately 29 cubic inches per degree Fahrenheit. If 100cubic inches of bypass gas pass through the low-flow control valve 22 orthe high-flow control valve 23 during each odorizer pulse, an extrapulse will be needed for approximately every three degrees change inodorant temperature to compensate for the change in volume of the liquidodorant.

As described above, operation of the PLC 30 is simplified by makingseveral calculations ahead of time, and storing into the PLC 30 theresulting data needed to properly operate the odorizer. For example,calculation of bypass gas flow rates through each of the low-flowcontrol valve 22 and the high-flow control valve 23 need only beperformed once, unless the valve is changed. Further, the requiredamount of bypass gas that must be released for each pulse will notchange unless the odorant is changed or the desired ratio of odorant togas is changed. Because these parameters change so rarely as to beconsidered fixed, this calculation can be performed in advance and theresult programmed into the PLC 30.

In order to perform the calculations to determine the dwell time of eachof the low-flow control valve 22 and the high-flow control valve 23,several items of information are needed. These include the flowcharacteristics of each of the low-flow control valve 22 and thehigh-flow control valve 23, generally provided from the manufacturer forgiven pressure drops, and how often pulses are needed, in terms of cubicfeet per pulse of gas flowing through the main pipeline 14. Also neededis the rate of odorization, in pounds per million cubic feet or othermass per volume ratio. Finally, the solubility of the odorant in naturalgas must be known, along with the operating pressures for the system,including pressure corrections such as operating atmospheric pressure.The pre-calculated dwell time for the low-flow control valve 22 may bethe same or different than the pre-calculated dwell time for thehigh-flow control valve 23. In an embodiment, the low-flow control valve22 has a longer pre-calculated dwell time than the high-flow controlvalve 23.

Using the solubility factors, pressures, and required pulse rate, knowncalculations provide the volume of saturated gas that will be needed foreach pulse of the odorizer. For example, assuming gas line pressure of230 psig, BP CAPTAN odorant having a solubility factor of 0.7, and anodorizer pulse every 500 cubic feet of gas flow, approximately 300 cubicinches of saturated gas must be added in each pulse to give anodorization rate of 0.75 pounds per million standard cubic feet. Inother words, if the odorizer is pulsed once every 500 cubic feet, eachpulse must provide 0.000375 pounds of odorant in the pulse of saturatedgas, which is approximately 300 cubic inches under the conditions of theexample.

Once the pulse volume is known, the required dwell time per pulse isdetermined from the flow rate of each of the low-flow control valve 22and the high-flow control valve 23. For this example, if the low-flowcontrol valve 22 allows 400 cubic inches per second to flow underoperating conditions, the dwell time of the low-flow control valve 22 is0.75 seconds. Under operating conditions, a 0.75 second pulse allowsenough saturated gas to flow into the line to properly odorize 500 cubicfeet of gas. Other conditions and requirements and conditions will, ofcourse, give different results.

Once these calculations are made, the volume for which a pulse will bemade, and the dwell time for each pulse are loaded in to the PLC 30. Ifconditions change, the PLC 30 can be reprogrammed by running thecalculation again and loading the new control data into the PLC 30. Thiscan be performed remotely, by a portable computer connected directly tothe PLC 30 on site, or by entering the new data directly at the PLCkeypad.

It will be appreciated by those skilled in the art that the describedsystem provides for improved operation. The system is reliable becausethere are no injection pumps to fail. The system is flexible becausechanges in operating conditions can be accommodated by changing thefrequency of pulsing the bypass system, changing the dwell time of thelow-flow control valve 22 and/or the high-flow control valve 23, andallowing for either the low-flow control valve 22 or the high-flowcontrol valve 23 (or both) to be opened. Because of its operationalsimplicity, the PLC 30 can be programmed in a more complex manner totake into account variables such as temperature changes in the liquidodorant 72 and valve opening duration.

The described system allows for remote control and monitoring of the gasodorization system 10. As previously described, in addition to on-sitecontrol through the keypad or by a direct link with a laptop computer,the PLC 30 may be reprogrammed by a remote computer through thecommunications link. In addition, operational data stored within the PLC30 may be read remotely by the computer. This allows for centralmonitoring and control of a large number of odorizers located in widelyseparated locations. Normal operating data as well as alarm data can begathered remotely, so that actual operator presence at the odorizer sitecan be minimized.

Further, the PLC 30 need not even be located at the site where odorantis injected into the gas line. Instead, the various valves and sensorscan be connected to actuators that are remotely controlled, either overa physical connection such as a direct communications line, or by remoteradio control. Such an arrangement could be useful, for example, when itis desired to control a large number of odorizers from a centrallocation. Instead of using separate PLCs at each location, a larger,central computer can be programmed to monitor all of the odorizerssimultaneously and control each of them in the same manner as describedabove. Including communications in the odorizer system allows forsimpler central control and programming.

It is specifically intended that the present disclosure not be limitedto the embodiments and illustrations contained herein, but includemodified forms of those embodiments including portions of theembodiments and combinations of elements of different embodiments ascome within the scope of the following claims.

We claim:
 1. A method for odorizing natural gas in a pipeline,comprising the steps of: providing liquid odorant in a tank; measuringgas flow through the pipeline; measuring a valve opening duration of ahigh-flow control valve in a bypass line and a low-flow control valve inthe bypass line; comparing the gas flow with a target flow count;comparing the valve opening duration with a target valve openingduration; opening the high-flow control valve when (i) the gas flow isequal to or greater than the target flow count and (ii) the valveopening duration is equal to or greater than the target valve openingduration, to allow a first known volume of gas, saturated with odorant,to flow from the odorant tank into the pipeline; wherein the high-flowcontrol valve is opened for a dwell time proportional to the first knownvolume of saturated gas; and opening the low-flow control valve when (i)the gas flow is equal to or greater than the target flow count and (ii)the valve opening duration is less than the target valve openingduration, to allow a second known volume of gas, saturated with odorant,to flow from the odorant tank into the pipeline; wherein the low-flowcontrol valve is opened for a dwell time proportional to the secondknown volume of saturated gas.
 2. The method of claim 1 furthercomprising the steps of: opening the high-flow control valve and thelow-flow control valve when (i) the gas flow is equal to or greater thanthe target flow count and (ii) the valve opening duration is greaterthan the target valve opening duration, to allow a third known volume ofgas, saturated with odorant, to flow from the odorant tank into thepipeline; wherein the high-flow control valve and the low-flow controlvalve are opened for a dwell time proportional to the third known volumeof saturated gas.
 3. The method of claim 1 further comprising the stepsof: sensing a temperature of the liquid odorant in the tank; andmodifying operation of the high-flow control valve and the low-flowcontrol valve in the bypass line to compensate for changes in thetemperature of the liquid odorant.
 4. The method of claim 1 furthercomprising the steps of: sensing whether gas flows through the bypassline when either of the high-flow control valve and the low-flow controlvalve is opened; and closing a master shutdown valve to block the bypassline if no gas flow is sensed through the bypass line when either of thehigh-flow control valve and the low-flow control valve is opened.
 5. Themethod of claim 1 further comprising the steps of: sensing whether gasflows through the bypass line when either of the high-flow control valveand the low-flow control valve is opened; and closing a master shutdownvalve to block the bypass line if gas flow is sensed through the bypassline when both the high-flow control valve and the low-flow controlvalve are closed.
 6. The method of claim 1 further comprising the stepof exchanging diagnostic information with a remote computer over a datalink.